1. Field of the Invention
The present invention relates generally to an air/fuel mixture ratio control system for an internal combustion engine. More specifically, the invention relates to a learning control system for controlling air/fuel ratio in a fuel injection internal combustion engine, which air/fuel ration control includes lambda (.gamma.) control for performing FEEDBACK or CLOSED LOOP control on the basis of oxygen concentration contained in an exhaust gas. Further particularly, the invention relates to an air/fuel ratio learning control system which can precisely adjust air/fuel ratio depending upon density of air to be introduced for forming the air/fuel mixture.
2. Description of the Background Art
In the recent years, there have been proposed various air/fuel control systems for internal combustion engines. Some of the recently developed air/fuel ratio control systems incorporate a learning control feature to continuously update a correction coefficient for correcting a basic fuel injection amount based on oxygen concentration in an exhaust gas in order to maintain the air/fuel ratio at a stoichiometric value. In the case of the air density dependent air/fuel ratio, the correction coefficient may be uniformly updated based on an oxygen concentration indicative sensor signal value (hereafter O.sub.2 sensor signal) regardless of the engine driving range, in theory. However, in practice, because of tolerance in fuel injection valves, throttle body and other engine components, which causes deviation between an arithmetically obtained basic fuel injection amount and practically required fuel injection amount, uniformly updating or learning of the correction coefficient regardless of engine driving range is practically not possible. By this, it is practically required to set a learnt correction coefficient for each respective engine driving range.
In view of this, learning control systems with a FEEDBACK control feature for controlling air/fuel ratio have been recently proposed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-90944 and the Japanese Patent First Publication (Tokkai) Showa No. 61-190142. In the disclosed system, a basic fuel injection amount is derived on the basis of a preselected basic fuel injection control parameter or parameters, such as intake air flow rate, engine revolution speed and so forth. The basic fuel injection amount thus derived is modified employing a feedback correction coefficient which is derived on the basis of an oxygen sensor in an exhaust system and composed of a proportional (P) component and an integral (I) component. By modifying the fuel injection amount on the basis of the feedback correction coefficient, the air/fuel ratio can be FEEDBACK controlled toward a stoichiometric value. Furthermore, the disclosed system derives a learnt correction coefficient with respect to mutually distinct engine operation ranges. In practice, the learnt correction coefficient is determined by deriving a difference between the feedback correction coefficient and a predetermined reference value. This learnt correction coefficient is used in OPEN LOOP mode air/fuel ratio control to derive the fuel injection amount The learnt correction coefficient may also be used in the FEEDBACK or CLOSED LOOP mode air/fuel ratio control together with the feedback correction coefficient.
Such system assures performance of the air/fuel ratio control in the FEEDBACK mode operation to maintain the air/fuel ratio precisely at the stoichiometric value. Furthermore, the learnt correction coefficient may serve to maintain a desired air/fuel ratio even in an OPEN LOOP mode operation.
However, in the aforementioned type of learning control system, a drawback may be encountered in an engine driving condition where the engine driving or operation range frequently fluctuates. For example, in hill or mountain climbing, the air/fuel ratio control is held in a transition mode condition between the FEEDBACK mode and the OPEN LOOP mode to too frequently change the engine driving range to update the learnt correction coefficient during FEEDBACK mode operation. Therefore, the learnt correction coefficient may not reflect the instantaneous air density. This causes a delay in the FEEDBACK mode control after the driving condition returns to a stable state satisfying the FEEDBACK condition. Furthermore, in the OPEN LOOP control, the air/fuel ratio tends to deviate far from the stoichiometric value to degrade drivability.